Technical Field
The disclosure relates to a memory structure, and particularly to a phase change memory (PCM) structure.
Description of the Related Art
Phase-change based memory materials, such as chalcogenide-based materials and similar materials, can be caused to change phase between an amorphous phase and a crystalline phase by application of electrical current at levels suitable for implementation in integrated circuits. In comparison with the crystalline phase, the amorphous phase is characterized by a higher electrical resistivity which can be readily sensed to indicate data. These properties may cause the chalcogenide-based materials suitable for serving as programmable resistive material to form nonvolatile memory circuits, which can be read and written with random access, e.g a phase-change memory devices.
A phase-change memory device can be read and written with random access by the transformations between the amorphous phase and the crystalline phase occurring on the chalcogenide-based materials disposed in the active area of the phase-change memory device. For example, the phase of the chalcogenide-based materials (phase-change materials) disposed in the active area of the phase-change memory device may be transformed from the amorphous phase to the crystalline phase when a low-voltage electric pulse is applied to the active area of the memory cells, whereby the resistance states of the memory cells involved in the phase-change memory device may be transformed either from an initial state or from a high resistance state to a low resistance state. The process of transforming resistance state form high to low is referred as a set process. Alternatively, the low resistance state of the memory cells may be transformed to the high resistance state again by applying a high-voltage electric pulse to the active area of the memory cells in order to transform the crystalline phase of the chalcogenide-based materials to the amorphous phase. The process of transforming resistance state form low to high is referred as a reset process.
However after a plurality numbers of read, set and reset operations are carried out, degradations of the phase-change materials may occur due to the electrical energy applied to the phase-change materials during the set/reset operations, the operation temperature and/or the arrangement of the memory cells. Such that the electrical properties, such as resistance state, relationship between the resistance and the current etc., of the phase-change material may shifted from an initial state.